Today, continuing our coverage of all the Web server controls in Visual Basic .NET, we got a look at a number of important Web server controls—image controls, image buttons, list boxes, drop-down lists, hyperlinks, and link buttons.
Image controls are designed simply to display images, which is more or less
it. They correspond to HTML <img> elements and don't support
events such as the Click event. Image buttons, on the other hand,
do support Click events, and you can use them to create clickable
image maps. When an image button is clicked, the exact location of the mouse
in the image is recorded and sent to the Click event's handler,
allowing you to take the action you want in response to "hotspot" clicks.
Today, we also took a look at Web server list boxes today. By default, Web
server list boxes support only single selections, and you can determine which
item in a list was selected using the SelectedIndex property, which
returns the index of the selected item; the SelectedItem property,
which returns the ListItem object corresponding to the selected item;
and the SelectedValue property, which returns the value of the selected
item's Value property.
You can support multiple selections in list boxes (but not drop-down list
boxes) by setting the SelectionMode property to ListSelectionMode.Multiple.
To determine which items are selected in a multiple selection list box, you
can loop over the Items collection of ListItem objects, checking
each item's Selected property to see if that item is selected.
You can add items to list controls at design time using the ListItem Collection
editor, or at runtime using the list control's Items collection's Add method.
We saw how to do both today.
We also took a look at drop-down lists here. These controls are similar to
list boxes, except that they omit the size HTML attribute, which makes
the browser display the control as a drop-down list. Drop-down lists display
an arrow button that, when clicked, opens a list from which the user can make
a selection. The user can only make single selections from drop-down lists,
however, because when you make a selection, the drop-down list closes immediately.
Similar to list boxes, you can make sure drop-down list box events, such as
the SelectedIndexChanged event, are sent back to the server as soon
as they occur by setting the AutoPostback property to True,
but you usually let the user make selections in list boxes and handle those
selections only after the user clicks a Submit button to send the page back
to the server.
Today, we also took a look at working with hyperlink controls. These controls
are handled by HTML <a> elements, and we saw that you can customize
hyperlink controls with the Text, NavigateUrl, ImageUrl,
and Target properties. However, hyperlink controls don't support
the Click event—for that, you need to use link buttons.
Link buttons appear at runtime just like hyperlinks, but in Visual Basic code
they function just like Web server buttons. Specifically, you can handle Click and Command events
for link buttons just as you can with Web server buttons. You use link buttons
when you want to execute code when the user clicks a hyperlink, not just let
the browser automatically navigate to a new URL. You saw several examples showing
how link buttons can be useful, including setting URL targets on-the-fly, according
to the time of day, or making other controls, such as labels, visible when
a hyperlink is clicked.
That's it for today's work—tomorrow, we're going to get more Web server power as we round off our Web application work by covering the final Web server controls—validation controls, calendars, and ad rotators.
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